How to avoid one code multiple times instead do that through some function? Shortcode, but I think shortcode is not used in main theme’s core files?

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You can use PHP’s includes like this:

include dirname(__FILE__).'/components/authorSocials.php';

but you should remember about providing the data to your component. Since the idea is that you will you use it in different places, not always the default values will be available and will be the right ones, so you should add some more code to this solution:

$user_id = assign_your_user_id_here();
include dirname(__FILE__).'/components/authorSocials.php';

then in your “component” do:

<?php 
    // Get the id of the post's author.
    $author_id = get_the_author_meta('ID', $user_id);

    .....
?>